Printing machines with a thermal head transfer the ink from a printing ribbon to the material to be printed by heating a number of points in the thermal print head to release the ink.
In a first generation of printing machines, the transfer ribbon transport means were driven by a motor in the take-up spool and were provided with various possible combinations of braking devices in the feed spool to control the amount of ribbon supplied.
During the printing stage it is crucial that the tension of the transfer ribbon be as constant as possible in order to obtain a high printing quality. In this sense, some manufacturers have developed moving devices that move to maintain the ribbon tension constant, thereby resulting in a relative motion between the print head and the ribbon, with the feed spool and take-up spool still.
This is the case in U.S. Pat. No. 5,975,777, which relates to a printing apparatus incorporating a shuttle that constitutes the transport element of the ribbon with respect to the thermal head during printing, while at least one of the spools remains still.
Other solutions used to maintain a constant ribbon tension involve acting on the various speeds of the feed and take-up spool motors.
These systems result complex and hard to manage, having poor precision as well as speed-related limitations determined by the inertia of the mechanical devices when these are set in motion, so that it is convenient to develop a device to solve the above-described problems.